(1) Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image held by an image holder such as a photosensitive drum, by using developer, and to an image forming apparatus provided with the developing device.
(2) Related Art
Image forming apparatuses, such as copiers or printers, are provided with a developing unit which houses a developer containing carrier and toner and develops an electrostatic latent image held by a photosensitive drum.
As one example of the developing method, there is known a trickle developing method in which fresh replenishment carrier is gradually replenished into a housing provided in the developing unit from an inlet of the housing, the developer housed in the housing is cyclically transported within the housing by a transport screw, and while the cyclical transport is performed, an excessive amount of developer, which is generated by the replenishment of carrier, is ejected to outside from an outlet provided in the housing as it overflows.
In the trickle developing method, old carrier contained in the developer in the housing is gradually replaced with fresh carrier, substantially preventing deteriorated carrier from remaining in the housing. This restricts the deterioration of the developer and realizes the high-quality image.
One problem of the developing unit adopting the trickle developing method is that the amount of developer housed in the housing is apt to vary depending on the image formation conditions.
That is to say, since the developer is transported to the outlet by the transport force that is generated as the transport screw is rotated, the amount of ejected developer depends on the rotation speed of the transport screw. For example, when the transport screw rotates at a high speed, a large amount of developer is transported per unit time to the outlet, and a large amount of developer is ejected. Conversely, when the transport screw rotates at a low speed, a small amount of developer is ejected.
Copiers or the like having the developing unit are typically provided with various copy modes from which selection can be made, such as the color mode and the monochrome mode. Many of the various modes are set to different system speeds. For example, when the system speed in the color mode is set as the standard speed, the system speed in the monochrome mode is set to be higher than the standard speed, from the view point of the copying productivity.
The rotation members, such as the photosensitive drum provided in the copier and the transport screw provided in the developing unit, are typically controlled to rotate at a speed corresponding to the system speed in each mode. That is to say, when the system speed is changed, the rotation speed of the transport screw is changed as well.
For example, when the copy mode is switched from the color mode to the monochrome mode, the system speed is switched from a low speed to a high speed, and the transport screw is rotated at a high speed.
When the rotation speed of the transport screw is switched from low to high, the amount of developer transported by the transport screw per unit time increases, and the amount of developer ejected from the outlet increases. When the amount of developer ejected from the outlet increases, the total amount of developer in the housing decreases that much, namely, a variation occurs in the amount of developer.
When the total amount of developer in the housing decreases as the transport screw rotates at a high speed, the liquid surface of the developer is lowered. When this state continues, the shortage of developer supply occurs, and the image quality is apt to be deteriorated due to reduction in the concentration during the developing process.
There is the opposite case in which the rotation speed of the transport screw is switched from high to low. For example, when the mode is switched to the mode for using the thick paper when the regular paper has been set as the standard among various types of paper for use, the system speed is switched to a lower speed than the standard speed since the fixing process for thick paper requires a larger amount of heat than for regular paper.
In that case, the transport screw is rotated at a low speed, and the amount of developer transported by the transport screw per unit time and the amount of ejected developer are decreased compared to the state before the switching. When the amount of ejected developer is decreased, the total amount of developer in the housing is increased that much. When this state continues, the amount of developer in the housing is increased excessively, and the driving torque of the transport screw is increased, which imposes the load on the driving motor and may cause the transport screw to rotate unevenly or cause the developer to overflow from the housing.